1. Field of the Invention
This invention relates to a transparent electrode, a patterning method for the same, and a manufacturing method for a semiconductor element of the same formed on a transparent substrate used in an integrated type photovoltaic device, a liquid crystal display device, and an organic EL device or the like.
2. Description of the Prior Art
A photovoltaic device comprising an amorphous semiconductor of amorphous silicon, amorphous silicon carbide, amorphous silicon germanium or the like has been developed as a solar cell device because it is manufactured at low cost and easily increases in size.
Explanation of a general amorphous photovoltaic device is made by referring to FIG. 35. A photovoltaic element 100 of an amorphous semiconductor is formed by laminating a transparent electrode 102, a photovoltaic conversion layer 103 of a lamination of p-, i-, and n-type amorphous semiconductor layers 103p, 103i, 103n, and a rear surface metal electrode 104 in this order on a glass substrate 101.
Tin oxide (SnO2) and ITO (Indium Tin Oxide) are used conventionally as transparent conductive material for forming the transparent electrode 102. In recent years, zinc oxide (ZnO) has been examined in order to reduce manufacturing cost and provide a photovoltaic element having high photovoltaic characteristics.
A photovoltaic element using ZnO is manufactured by the following processes. For example, a transparent electrode 102 of ZnO is formed on a glass substrate 101 by sputtering, and a photovoltaic conversion layer 103 formed of a lamination of a p-type layer 103p of approximately 150 xc3x85 of p-type amorphous silicon carbide, an i-type layer 103i of approximately 4000 xc3x85 of i-type amorphous silicon, and an n-type layer 103n of approximately 200 xc3x85 of n-type amorphous silicon is formed by plasma CVD. Then, a rear surface metal electrode 104 of silver (Ag) is sequentially laminated by sputtering to manufacture the photovoltaic element 100.
Photovoltaic elements including transparent electrodes 102 of ZnO of various thickness are manufactured and the photovoltaic conversion efficiency of them are measured. The results are shown in FIG. 36. As shown in a characteristic diagram of FIG. 36, high photovoltaic conversion efficiency over 10.5% is obtained when a thickness of the transparent electrode 102 is approximately 2100-5000 xc3x85.
The photovoltaic element using the transparent electrode of ZnO can provide high photovoltaic conversion efficiency.
The photovoltaic device comprising the amorphous semiconductor has an integrated structure so as to take out a high voltage from a single substrate.
In order to form the integrated structure, it is necessary to separate the transparent electrode film, the amorphous semiconductor layer, and the metal electrode film on the glass substrate. A laser patterning method is used for the separation.
A structure of a laser patterning device is explained by referring to FIG. 37. A laser 11 emitted from an Nd:YAG laser oscillating device 10 changes its direction at a reflection mirror 12, is condensed by a condensing lens 13, and irradiates a region to be processed of an object to be processed 20 mounted on a moving table 14 of an XYZ stage. A pattern is controlled by moving the moving table 14 with the object to be processed 20 mounted thereon.
In patterning a transparent electrode by using this device, the transparent substrate 21 with the transparent electrode film 22 formed on the whole surface thereof as the object to be processed is mounted on the moving table 14. The moving table 14 is controlled to move in XYZ directions so that the region to be processed of the transparent electrode 22 is eliminated by laser as shown in FIG. 38.
In the meantime, when the ZnO film formed by sputtering as the transparent electrode on the transparent substrate (a glass substrate) 21 is laser-patterned by an Nd:YAG laser (a wavelength of 1064 nm, power 10W, and a beam diameter 50 xcexcm), phenomena particular to the ZnO film, which is not observed when using material such as SnO2 and ITO as the transparent electrode, are found. That is, (1) volume expansion of a laser irradiated end part, (2) a crack of a laser irradiated end part, (3) peeling of the ZnO film from the glass substrate, (4) diffusion of a ZnO forming element into the glass substrate, or the like.
These phenomena seem to complicatedly relate to a crystalline structure, heat conductivity, a surface tension in melting or the like of ZnO material. These phenomena may result in short-circuit between electrodes of the photovoltaic device, degraded reliability of the photovoltaic device, separation failure of the ZnO film (insulation failure between adjacent photovoltaic elements), and these problems obstruct commercialization of the ZnO material. The photovoltaic element using the transparent electrode of the ZnO film can not provide sufficient characteristics as an integrated type photovoltaic device while it can provide high photovoltaic conversion efficiency.
It is generally known that the ZnO material has moisture absorption property. Therefore, when the ZnO film is retained in an atmosphere over a day, moisture in the atmosphere penetrates from a surface of the ZnO film and solid state properties change, resulting in significant failure of laser patterning.
FIG. 39 is results of temperature distribution simulation in laser patterning; an Nd:YAG laser (wavelength 1064 nm, power 10W, a beam diameter 50 xcexcm) is irradiated to an aluminum dope ZnO film(7500 xc3x85 in thickness) formed on a glass substrate by sputtering. In this simulation, it is assumed that laser energy injected to the ZnO film (laser energy excluding reflection and transmission loss) is converted into heat energy.
The laser-irradiated ZnO film is required to be evaporated (scattered) with the temperature reaching over the melting point and be completely eliminated so as to obtain complete electrical insulation by laser patterning. The ZnO film is required to melt to an interface of the glass and the ZnO film completely. In this case, a temperature of the ZnO film on a surface side reaches over the melting point and is in a high temperature state.
The excessive energy not only is converted into kinetic energy of a ZnO molecule but also moves to a periphery of a laser irradiated part by heat conduction, resulting in following heat damages; (1) volume expansion of the laser irradiated end part, (2) a crack of the laser irradiated end part, (3) peeling of the ZnO film from the glass substrate, (4) diffusion of the ZnO forming element in the glass substrate.
These problems may cause short-circuit passing through a semiconductor layer later formed, or a crack generated in the ZnO film in other processes may cause loss of a part of the film and degradation of reliability. In addition, diffusion of component elements of the ZnO film to the glass substrate causes separation failure (electrical insulation failure).
The above simulation is made in assumption that all laser energy is converted into heat energy. However, in actually dividing the ZnO film into a plurality of electrodes by laser patterning, absorption of a laser is insufficient when the ZnO film is thin, resulting in incomplete electrical separation.
A ZnO film having various thickness is formed on a glass substrate, and is divided into two electrodes by laser patterning. Then resistance between the two electrodes is measured. FIG. 40 is a characteristic view showing relations between a thickness of the ZnO film and yields of non-defective item having resistance between the electrodes not less than 10Mxcexa9.
A separation width of the two electrodes is approximately 100 xcexcm as in the case of using for a photovoltaic device. It is found that high yields of over 90% can be obtained when a thickness of the ZnO film is larger than 5000 xc3x85 but as a thickness becomes small, the yield declines. When the thickness is smaller than 4500 xc3x85, yields is as low as under 40%.
The measurements show whether the film is completely separated depending on resistance between the electrodes. When the thickness is large, as shown in FIG. 39, a temperature on a surface of the ZnO film is far higher than the melting point and deficiency particular to the ZnO film may occur. Therefore, when the ZnO film with merely a large thickness is used as a transparent electrode of the photovoltaic device, degradation of characteristics and reliability may occur.
This invention was made to use a transparent electrode film for a photovoltaic device of a ZnO film (gallium dope ZnO and aluminum dope ZnO) film which is manufactured at low cost as compared with conventionally used SnO2, ITO or the like. In addition, this invention was made to provide a transparent electrode and a manufacturing method thereof capable of preventing phenomena (heat damages, insulation failure or the like) particular to the ZnO film appearing in a laser patterning process for forming the integrated type photovoltaic device.
A transparent electrode according to this invention is formed by patterning a transparent electrode film formed on a transparent substrate with a laser, and comprises a transparent electrode film having a lower melting point than that of a ZnO film and the ZnO film laminated in this order on the transparent substrate.
As described above, the transparent electrode having a lower melting point is provided on a side apart from the surface to be irradiated with a laser, thus injection of excessive laser energy higher than the melting point of the material becomes unnecessary, resulting in suppression of heat damage (volume expansion, a crack) on the end part to be irradiated with a laser.
An ITO film may be used as a transparent electrode film having a lower melting point than that of the ZnO film.
Because the melting point of the ITO film is lower than that of the ZnO film, excessive laser energy higher than the melting point of the material becomes unnecessary, resulting in suppression of heat damage of a laser irradiated end part.
The ITO film may be 100-1000 xc3x85, and the ZnO film may be 2500-5000 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
The transparent electrode film having the lower melting point than that of the ZnO film is a metal thin film.
The metal thin film may be formed of silver and is 50-300 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
The metal thin film is formed of aluminum and is 50-100 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
A moisture proof layer containing nitrogen may be formed on a surface of the ZnO film.
The moisture proof layer prevents moisture from penetrating from a surface of the ZnO film so that workability of laser-patterning is not degraded. In addition, long-term reliability of the ZnO film is improved.
In a patterning method for a transparent electrode according to this invention, a transparent electrode film having a lower melting point than that of a ZnO film is formed on a transparent substrate, a ZnO film is formed on the transparent electrode film, and regions to be irradiated of the transparent electrode film and the ZnO film are eliminated and patterned by laser irradiation.
As described above, the transparent electrode having a lower melting point is provided on a side apart from the surface to be irradiated with a laser, thus injection of excessive laser energy higher than the melting point of the material becomes unnecessary, resulting in suppression of heat damage (volume expansion, a crack) on the end part to be irradiated with a laser.
A moisture proof layer containing nitrogen is formed on a surface of the ZnO film.
The moisture proof layer prevents moisture from penetrating from a surface of the ZnO film so that workability of laser-patterning is not degraded. In addition, long-term reliability of the ZnO film is improved.
An ITO film as the transparent electrode film having the lower melting point than that of the ZnO film is formed by sputtering.
The transparent substrate is a glass substrate and a crystallized glass layer is provided between the glass substrate and the ITO film.
The crystallized glass layer prevents the ZnO component element from diffusing to the glass substrate in laser-patterning.
The ITO film may be 100-1000 xc3x85, and the ZnO film may be 2500-5000 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
The transparent electrode film having the lower melting point than that of the ZnO film may be a metal thin film.
The metal thin film is formed of silver and is 50-300 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
The metal thin film is formed of aluminum and is 50-100 xc3x85.
With this structure, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.
A manufacturing method for a semiconductor element according to this invention comprises a process for forming a transparent electrode film having a lower melting point than that of a ZnO film on a transparent substrate and forming a ZnO film on the transparent electrode film, a process for forming a plurality of transparent electrodes on the transparent substrate by irradiating laser on the ZnO film and eliminating and patterning regions to be irradiated of the transparent electrode film and the ZnO film, a process for forming a photovoltaic conversion layer of an amorphous semiconductor on the transparent substrate with the plurality of divided transparent electrodes included, and a process for forming a highly reflective conductive film on the photovoltaic conversion layer.
The manufacturing method for the semiconductor element according to this invention further comprises a process for separating the previously formed photovoltaic conversion layer into a plurality of photovoltaic conversion layers by laser irradiation, and a process for dividing into a plurality of electrodes by laser irradiation to the previously formed highly reflective conductive film.
Through these processes, good conversion efficiency and a good release voltage are obtained from a photovoltaic device using this transparent electrode.